This invention relates to a wingsail and more particularly to a wingsail with a flexible flap that responds to air flow so as to reduce rolling moment of the wingsail.
In yacht racing the geometry of the wind direction and the angle of attack of the sails is such that aerodynamic lift on the wing is resolved into forward thrust on the yacht. The wind flow also establishes a rolling moment that can cause capsizing. It is particularly important to avoid increases in rolling moment at high wind speed to avoid dangerous capsizing.
Several researchers have considered the optimization of spanwise loading on a wing, subject to different constraints. Jones (1) calculated the optimum spanwise lift distribution for a wing subject to a constraint on lift and root bending moment. Tan and Wood (2) applied these ideas to determine the optimum spanwise lift distribution for a yacht sail subject to a constraint on the rolling moment while maximizing forward thrust. Subsequent researchers, such as Junge et al. (3) and Sneyd and Sugimoto (4) extended the analysis to include spanwise variation of wind strength and direction and boat heel. All of these analyses confirm the importance of maximizing lift and/or forward thrust while constraining rolling moment. In the analysis of a yacht, the geometry of the wind direction relative to the yacht direction is such that aerodynamic lift on the wing provides a component of forward thrust on the yacht. Thus we will occasionally use the terms lift and thrust interchangeably.
These prior art analyses focused on the design of a wing or a fixed sail planform optimized to operate at a given wind speed. As sailing has moved to the use of wingsails, the analysis of the sail overlaps with traditional aerodynamics. However, unlike an aircraft wing which is designed to operate at a gives flight speed, and is equipped with devices such as flaps for lower landing speeds, a racing yacht operates over a wide range of wind speeds. Typical values would range from 10-30 knots, at which point the race would be called off.
The present invention is a wingsail with a substantially rigid airfoil section having a leading and a trailing edge. A flap is attached to the trailing edge through a torsion fitting having a torsional stiffness along the span of the rigid airfoil section selected to control flap motion with respect to the rigid airfoil section under aerodynamic loading to control rolling moment of the wingsail. In preferred embodiments, the torsional stiffness is constant along the span or the torsional stiffness varies along the span. The flap may be segmented or unitary or both. The rigid airfoil section and the flap may have a constant chord along the span or a varying chord along the span.
It is therefore an object of the invention to provide a wingsail having a flap portion to control rolling moment of the wingsail.